Parietal bone

The parietal bones (/pəˈraɪ.ɪtəl/) are two bones in the humanskull which, when joined together at a fibrous joint, form the sides and roof of the cranium. Each bone is roughly quadrilateral in form, and has two surfaces, four borders, and four angles. It is named from the Latin paries (-ietis), wall.

Surfaces

External

The external surface [Fig. 1] is convex, smooth, and marked near the center by an eminence, the parietal eminence (tuber parietale), which indicates the point where ossification commenced.

Crossing the middle of the bone in an arched direction are two curved lines, the superior and inferior temporal lines; the former gives attachment to the temporal fascia, and the latter indicates the upper limit of the muscular origin of the temporalis.

Above these lines the bone is covered by the galea aponeurotica (epicranial aponeurosis); below them it forms part of the temporal fossa, and affords attachment to the temporalis muscle.

Internal

The internal surface [Fig. 2] is concave; it presents depressions corresponding to the cerebral convolutions, and numerous furrows (grooves) for the ramifications of the middle meningeal artery; the latter run upward and backward from the sphenoidal angle, and from the central and posterior part of the squamous border.

Near the groove are several depressions, best marked in the skulls of old persons, for the arachnoid granulations (Pacchionian bodies).

In the groove is the internal opening of the parietal foramen when that aperture exists.

Figure 1 : Left parietal bone. Outer surface.

Figure 2 : Left parietal bone. Inner surface.

Left parietal bone (shown in green). Animation stops for a few seconds at inner and outer surface.

Borders

The sagittal border, the longest and thickest, is dentated (has toothlike projections) and articulates with its fellow of the opposite side, forming the sagittal suture.

The frontal border is deeply serrated, and bevelled at the expense of the outer surface above and of the inner below; it articulates with the frontal bone, forming half of the coronal suture. The point where the coronal suture intersects with the sagittal suture forms a T-shape and is called the bregma.

The squamous border is divided into three parts: of these:

the anterior is thin and pointed, bevelled at the expense of the outer surface, and overlapped by the tip of the great wing of the sphenoid;

the middle portion is arched, bevelled at the expense of the outer surface, and overlapped by the squama of the temporal;

the posterior part is thick and serrated for articulation with the mastoid portion of the temporal.

The occipital border, deeply denticulated (finely toothed), articulates with the occipital bone, forming half of the lambdoid suture. That point where the sagittal suture intersects the lambdoid suture is called the lambda, because of its resemblance to the Greek letter.

Skull seen from top. Sagittal suture separates left and right parietal bone.

Angles

The frontal angle is practically a right angle, and corresponds with the point of meeting of the sagittal and coronal sutures; this point is named the bregma; in the fetal skull and for about a year and a half after birth this region is membranous, and is called the anterior fontanelle.

The sphenoidal angle, thin and acute, is received into the interval between the frontal bone and the great wing of the sphenoid. Its inner surface is marked by a deep groove, sometimes a canal, for the anterior divisions of the middle meningeal artery.

The occipital angle is rounded and corresponds with the point of meeting of the sagittal and lambdoidal sutures—a point which is termed the lambda; in the fetus this part of the skull is membranous, and is called the posterior fontanelle.

The mastoid angle is truncated; it articulates with the occipital bone and with the mastoid portion of the temporal, and presents on its inner surface a broad, shallow groove which lodges part of the transverse sinus. The point of meeting of this angle with the occipital and the mastoid part of the temporal is named the asterion.

Ossification

The parietal bone is ossified in membrane from a single center, which appears at the parietal eminence about the eighth week of fetal life.

Ossification gradually extends in a radial manner from the center toward the margins of the bone; the angles are consequently the parts last formed, and it is here that the fontanelles exist.

Occasionally the parietal bone is divided into two parts, upper and lower, by an antero-posterior suture.

In other animals

In non-human vertebrates, the parietal bones typically form the rear or central part of the skull roof, lying behind the frontal bones. In many non-mammalian tetrapods, they are bordered to the rear by a pair of postparietal bones that may be solely in the roof of the skull, or slope downwards to contribute to the back of the skull, depending on the species. In the living tuatara, and many fossil species, a small opening, the parietal foramen, lies between the two parietal bones. This opening is the location of a third eye in the midline of the skull, which is much smaller than the two main eyes.[1]

In dinosaurs

The parietal bone is usually present in the posterior end of the skull and is near the midline. This bone is part of the skull roof, which is a set of bones that cover the brain, eyes and nostrils. The parietal bones make contact with several other bones in the skull. The anterior part of the bone articulates with the frontal bone and the postorbital bone. The posterior part of the bone articulates with the squamosal bone, and less commonly the supraoccipital bone. The bone-supported neck frills of ceratopsians were formed by extensions of the parietal bone. These frills, which overhang the neck and extend past the rest of the skull is a diagnostic trait of ceratopsians. The recognizable skull domes present in pachycephalosaurs were formed by the fusion of the frontal and parietal bones and the addition of thick deposits of bone to that unit.[2]

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The diploic veins are large, thin-walled valveless veins that channel in the diploë between the inner and outer layers of the cortical bone in the skull. They are lined by a single layer of endothelium supported by elastic tissue. They develop fully by the age of two years. The diploic veins drain this area into the dural venous sinuses. The four major trunks of the diploic veins found on each side of the head are frontal, anterior temporal, posterior temporal, and occipital diploic veins.

A headbutt (French coup de boule) is a targeted strike with the head, typically (when intentional) involving the use of robust parts of the headbutter's cranium as the area of impact. The most effective headbutts strike the most sensitive areas of an opponent, such as the nose, using the stronger bones in the forehead (frontal bone) or the back of the skull (occipital or parietal bone). It can be considered a quick, very effective but risky maneuver, as a misplaced strike can cause greater injury to the person delivering the headbutt than to the person receiving it. A headbutt does not have to be against another person's head, although this is usually the nearest and easiest target.

In the United Kingdom, a headbutt is sometimes referred to as a Glaswegian kiss. This is a tongue-in-cheek reference to the city of Glasgow's violent reputation.

Huehuecuetzpalli mixtecus is an extinct lizard from the Early Cretaceous (specifically the middle to late Aptian stage) Tlayúa Formation in Tepexi de Rodríguez, Central Mexico. Although it is not the oldest known lizard, Huehuecuetzpalli may be the most basal member of Squamata (the group that includes lizards and snakes), making it an important taxon in understanding the origins of squamates. It may or may not be a basal member of Iguania, a large clade of lizards that traditionally includes the iguanas and their close relatives, chameleons, and agamids: if it is an iguanian, H. mixtecus represents the earliest major offshoot of the squamate evolutionary tree.

Unique characteristics (autapomorphies) of Huehuecuetzpalli include a long pair of premaxilla bones at the tip of the upper jaw that contributes to an elongated snout and the apparent retraction of the external nares or nostril openings. At the top of the skull, a small rounded postfrontal bone and a hole called the parietal foramen between the junction of the frontal bone and the parietal bone (the frontoparietal suture) suggest affinities with iguanians, but the retention of divided premaxillae, amphicoelous vertebrae (vertebrae that are concave at both ends), thoracolumbar intercentra (bones between the vertebrae of the back), an entepicondylar foramen in the humerus (upper arm bone), and a second distal tarsal bone in the foot supports the hypothesis that Huehuecuetzpalli is a basal squamate.

Lubbockichthys is a genus of ray-finned fish from the Indo-Pacific region which belongs to the subfamily Pseudoplesiopinae, part of the family Pseudochromidae, the dottybacks. The species in this genus have small cycloid scales throughout their lives; some of their head bones have a weakly honeycombed surface; and the parietal bone encloses the rear section of the supratemporal laterosensory canal.The name of this genus honours the Cambridge University ichthyologist, Roger Lubbock (1951-1981) in recognition of his work on the taxonomy of the subfamily Pseudoplesiopinae.

The middle meningeal artery (Latin: arteria meningea media) is typically the third branch of the first part (retromandibular part) of the maxillary artery, one of the two terminal branches of the external carotid artery. After branching off the maxillary artery in the infratemporal fossa, it runs through the foramen spinosum to supply the dura mater (the outermost meninges) and the calvaria. The middle meningeal artery is the largest of the three (paired) arteries that supply the meninges, the others being the anterior meningeal artery and the posterior meningeal artery.

The anterior branch of the middle meningeal artery runs beneath the pterion. It is vulnerable to injury at this point, where the skull is thin. Rupture of the artery may give rise to an epidural hematoma. In the dry cranium, the middle meningeal, which runs within the dura mater surrounding the brain, makes a deep indention in the calvarium.

The middle meningeal artery is intimately associated with the auriculotemporal nerve, which wraps around the artery making the two easily identifiable in the dissection of human cadavers and also easily damaged in surgery.

Nägele's obliquity is the presentation of the anterior parietal bone to the birth canal during vaginal delivery with the biparietal diameter being oblique to the brim of the pelvis. The synonym for this presentation is anterior asynclitism. It was first described in 1777 by German Karl Nägele.

A neck frill is the relatively extensive margin seen on the back of the heads of reptiles with either a bony support such as those present on the skulls of dinosaurs of the suborder Marginocephalia or a cartilaginous one as in the frill-necked lizard.

In technical terms, the bone-supported frill is composed of an enlarged parietal bone flanked by elongated squamosals and sometimes ringed by epoccipitals, bony knobs that gave the margin a jagged appearance. In the early 1900s, the parietal bone was known among paleontologists as the dermosupraoccipital. The feature is now referred to as the parietosquamosal frill. In some genera, such as Triceratops, Pentaceratops, Centrosaurus and Torosaurus, this extension is very large. Despite the neck frill predominantly being made of hard bone, some neck frills are made of skin, as is the case with the frill-necked lizard of today that resides in Australia. The use of the neck frill in dinosaurs is uncertain; it may have been used for thermoregulation or simply as a defense mechanism. Indeed, during battles for territory, competing Triceratops crashed heads together with their elongated horns and the neck frill may have been employed as a kind of shield, protecting the rest of the animal from harm. However, usage of the neck frill in modern reptiles is better documented. Two chief and disparate examples are the horned lizards (genus Phrynosoma) with a bony frill, and the frill-necked lizard (genus Chlamydosaurus) with a cartilaginous frill. The frill-necked lizard's frill is mainly made up of flaps of skin, which are usually coloured pink, supported by cartilaginous spines. Similar to the portrayal of the dinosaur Dilophosaurus in Steven Spielberg's Jurassic Park, frill-necked lizard puff out these neck frills on either side of its head when threatened. The lizards often raise their frills when battling for territory or when coming into contact with another lizard, especially during mating season. There is, however, no evidence that suggests that Dilophosaurus neither had nor didn't have the same abilities, as many of its features in the Jurassic Park film were mostly fictional.

Numerous other animals of both modern and prehistoric times use both skin or bone protrusions to make themselves seem more threatening, attract mates or to thermoregulate. Examples of these are the usage of dewlaps and crests in lizards, dinosaurs and birds.

In human anatomy, the neurocranium, also known as the braincase, brainpan, or brain-pan is the upper and back part of the skull, which forms a protective case around the brain. In the human skull, the neurocranium includes the calvaria or skullcap. The remainder of the skull is the facial skeleton.

In comparative anatomy, neurocranium is sometimes used synonymously with endocranium or chondrocranium.

The parietal foramen is an opening for the parietal emissary vein, which drains into the superior sagittal sinus. Occasionally, a small branch of the occipital artery can also pass through it. It is located at the back part of the parietal bone, close to the upper or sagittal border. It is not always present, and its size varies considerably.

The parietal lobe is one of the four major lobes of the cerebral cortex in the brain of mammals. The parietal lobe is positioned above the temporal lobe and behind the frontal lobe and central sulcus.

The parietal lobe integrates sensory information among various modalities, including spatial sense and navigation (proprioception), the main sensory receptive area for the sense of touch (mechanoreception) in the somatosensory cortex which is just posterior to the central sulcus in the postcentral gyrus, and the dorsal stream of the visual system. The major sensory inputs from the skin (touch, temperature, and pain receptors), relay through the thalamus to the parietal lobe.

Several areas of the parietal lobe are important in language processing. The somatosensory cortex can be illustrated as a distorted figure – the homunculus (Latin: "little man"), in which the body parts are rendered according to how much of the somatosensory cortex is devoted to them. The superior parietal lobule and inferior parietal lobule are the primary areas of body or spacial awareness. A lesion commonly in the right superior or inferior parietal lobule leads to hemineglect.

The name comes from the parietal bone, which is named from the Latin paries-, meaning "wall".

Parietal scale refers to the scales of a snake which are on the head of the snake and are connected to the frontals towards the posterior. These scales are analogous to and take their name from the parietal bone which forms the roof and sides of the cranium in humans.

The squamosal suture, or squamous suture, arches backward from the pterion and connects the temporal squama with the lower border of the parietal bone: this suture is continuous behind with the short, nearly horizontal parietomastoid suture, which unites the mastoid process of the temporal with the region of the mastoid angle of the parietal bone.

Crossing the middle of the parietal bone in an arched direction are two curved lines, the superior and inferior temporal lines; the former gives attachment to the temporal fascia, and the latter indicates the upper limit of the muscular origin of the temporal muscle.

The Tusculum portrait or the Tusculum bust is one of the two main portrait types of Julius Caesar, alongside the Chiaramonti Caesar. Being one of the copies of the bronze original, the bust is dated to 50–40 BC and is housed in the permanent collection of the Museo d'Antichità in Turin, Italy. Made of fine grained marble, the bust measures 33 cm (1ft 1in) in height.

The portrait's facial features are consistent with those on coins struck in Caesar's last year, particularly on the denarii issued by Marcus Mettius. The bust's head is prolonged, forming a saddle shape which was caused by Caesar's premature ossification of the sutures between the parietal bone and the temporal bone. The portrait also exhibits dolichocephalia. According to several scholars, the Tusculum portrait is the only extant portrait of Caesar made during his lifetime.The Tusculum portrait was excavated by Lucien Bonaparte at the forum in Tusculum in 1825 and was later brought to Castello d'Aglie, though it was not recognised as a bust of Caesar until Maurizio Borda identified it in 1940. The portrait was exhibited in the Louvre alongside the Arles bust. There are three known copies of the bust, in the Woburn Abbey and in private collections in Florence and Rome.

Wormian bones, also known as intrasutural bones or sutural bones, are extra bone pieces that can occur within a suture (joint) in the skull. These are irregular isolated bones that can appear in addition to the usual centres of ossification of the skull and, although unusual, are not rare. They occur most frequently in the course of the lambdoid suture, which is more tortuous than other sutures. They are also occasionally seen within the sagittal and coronal sutures. A large wormian bone at lambda is often called an Inca bone (Os Incae), due to the relatively high frequency of occurrence in Peruvian mummies. Another specific Wormian bone, the pterion ossicle, sometimes exists between the sphenoidal angle of the parietal bone and the great wing of the sphenoid bone. They tend to vary in size and can be found on either side of the skull. Usually, not more than several are found in a single individual, but more than one hundred have been once found in the skull of a hydrocephalic adult.

Wormian bones are a marker for some diseases and important in the primary diagnosis of brittle bone disease: osteogenesis imperfecta.Wormian bones may also be seen in:

Pycnodysostosis

Osteogenesis imperfecta

Rickets

"Kinky-hair" Menke's syndrome

Cleidocranial dysostosis

Hypothyroidism and hypophosphatasia

Otopalatodigital syndrome

Primary acro-osteolysis

Down's syndromeThese causes can be remembered by the mnemonic "PORKCHOPS".

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